Arthropod defensive secretions

The literature on arthropod chemical defensive secretions is so thoroughly covered in reviews (472-482) that it will be discussed only briefly here. A recent and detailed book is that of Blum (68). 

The chemicals may be biosynthesized de novo or sequestered from dietary plants which contain compounds that may serve as precursors of the allomone or are toxic in themselves. 

The qualitative and quantitative composition of arthropod defensive secretions may vary according to age, sex, physiological status, instar, caste and season. 

There are two major types of defensive substances, those which are elaborated by special exocrine glands and those not strictly of glandular origin which are contained in the blood, gut or elsewhere in or on the body (472). 

The secretion may kill, as for example when HCN is discharged by Geophitus vittatus 483) it may repel as when isobutyric acid and 2-methylbutyric acid are emitted by Papilio machaon caterpillars 472) or it may mobilize predators as is the case of glomerin (131) and ho-moglomerin (132) which are given off by Glomeria marginata 484). 

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While arthropod defensive secretions often rely for their effect on well-known aliphatic acids, aldehydes, phenols, and quinones, there are many cases in which compounds capable of whetting the appetite of any natural products chemist are utilized. For example, steroids play a... 

Weatherston J, Percy JE (1970) Arthropod defensive secretions. In Beroza M (ed) Chemicals controlling insect behavior. Academic Press, New York, p 95... 

Arthropod defensive secretions may also be sequestered from food plants 68). Thus Ferguson and Metcalf 170) found that four species of Diabrotica beetles sequester cucurbitacins as their defense compounds. 

Weatherston, J., and J. Percy Arthropod Defensive Secretions. In M. Beroza ed.. Chemicals Controlling Insect Behavior, p. 95-144. New York Academic Press 1970. 

Component of arthropod defensive secretions isol. from the grasshopper Romalea microptera. Metab. of Streptothrix chromogena. Dienophile, H acceptor for Oppenauer oxidns., mild dehydrogenating agent. 

T. Eisner D. Alsop K. Hicks J. Meinwald, Defensive secretions of millipedes. In Arthropods Venoms S. Bettini, Ed. Springer Berlin, 1978 pp 41-72. 

Coccinelline.—Coccinelline (16) is an alkaloid isolated from the defensive secretion of the Coccinellidae. [l- C]Acetate has been found to be incorporated into this arthropod alkaloid with 16% of the activity confined to C-2 plus C-10. This is consistent with derivation via the polyketide (17) (or the alternative with the carboxy-group at what becomes C-10 of coccinelline). 

Examples of fatty acid-derived defensive compounds are widespread among arthropods. In the simplest cases, long-chain fatty acids themselves have been shown to serve as repellents that deter predators such as ants. °° A classic example is the defensive secretion of the whip scorpion, Mastigoproctus giganteus, which contains an aqueous mixture of acetic and caprylic acids. More recent examples include the mayolenes... 

Simple pyridine, pyrazine, and pyrrolo derivatives have been described from many different types of arthropods. Nicotine (177), nicotinamide, nicotinic acid, 2-pyrrolidone, N-methylpyrrolididone (178), 8-hydroxyquinoline (179), and 2-isobutyl-3-methoxypyrazine (180) have been detected in the defensive secretion and blood of larvae of the moth Lymantria dispar The nicotine isomer anabasine (181) and its congener anabaseine (182) have been identified as venom components in several ant species,whereas... 

Attygalle, A. B., McCormick, K. D., Blankespoor, C. L., Eisner, T., and Meinwald, J. (1993). Defense mechanisms of arthropods. 116. Azamacrolides A family of alkaloids from the pupal defensive secretion of a ladybird beetle Epilachna varivestis). Proc. Natl. Acad. Sci. USA 90, 5204-5208. 

CioH.jN, Mr 147.22, oil, bp. 100-103 °C(1 kPa),[a]i, -7.2° (CHCI3), a monoterpene alkaloid ( iridoids) from the east Asian creeper Actinidia polygama and other Actinidiaceae, Bignoniaceae, and Valerianaceae. It is a component of the defensive secretion of black beetles, ants, grasshoppers, and flies. Only the (5) enantiomer occurs in nature. A. exhibits antimicrobial properties and a deterrent effect against birds and arthropods , while cats are strongly attracted Synthesis There are various syntheses of the racemate and the unnatural (/I) enantiomer, but only three for the (5) enantiomer. The biosynthesis presumably proceeds from iridodial. 

Ut. Benini, Arthropod Venoms, Berlin Springer l977 Blum. Chemical E>efenses of Arthropods, New York Academic Press 1981 Whitman et al., in Evans Schmidt (eds.). Insect Defenses, p. 289-420, Albany SUNY Press 1990 see also defensive secretions. 

Arthropods synthesize an incredible diversity of natural products in their exocrine glands (3) which are utilized with great effectiveness to blunt the attacks of their adversaries. The defensive secretions of these invertebrates can often be delivered with great accuracy, thus ensuring that aggressive predators are subjected to the full impact of these secretory effronteries (A). Significantly, these exudates usually manifest their defensive efficacy as effective repellents and thus constitute the first line of defense of the prey species. The deterrent value of the natural products in these secretions is inestimable, since it can permit their producers to escape predators without injurious physical confrontations. This is especially important in encounters with ants, since these social insects can quickly launch devastating en masse attacks. 

A variety of studies has demonstrated that ants may be rapidly deterred by the defensive secretions of diverse arthropods on which they attempt to prey (5, 6). For example, many of the small and delicate species of thrips (Thysanoptera) produce anal secretions that are directed against the ants with which they frequently have encounters (7), Although the defensive secretions of very few thrips species have been analyzed, evaluation of the compounds present in a few of these exudates indicates that they are effective deterrents for ants (8, 9). These results further suggest that repellents for ants may be commonly encountered in the defensive secretions of a variety of arthropodous species. Furthermore, since ants have frequent antagonistic interactions with other species of ants, it could be anticipated a priori that these formicids would have evolved powerful ant deterrents themselves. 

Simple phenols that can be converted readily to quinones by enzymatic oxidation are used by arthropods as defensive secretions. The biosynthetic origin of the phenolic substrates is not known. Probably the most remarkable of these is the defense of bombardier beetles, of the genus Brachynus. A secretion as hot as 100°C is produced by a reaction among hydroquinone, a phenolic substrate, H2O2 and the enzyme catalase. A highly exothermic reaction occurs as hydroquinone is oxidized to benzoquinone, the major product of defense. The beetle, when endangered, discharges a hot explo-... 

Certain defensive secretions emitted by arthropods are especially effective since they are emitted as a mixture rather than as a single compound. Thus, the carabid beetle Helluomorphoides sp. secretion consists of formic acid and nonyl acetate. The former is an irritant while the ester acts as a penetrating promoting agent 472). 

Eisner, T., D. Alsop, K. Hicks, and J. Meinwald Defensive Secretions of Millipedes. In S. Bettini ed.. Handbook of Experimental Pharmacology vol. 48, Arthropod Venoms , p. 41-72, Berlin Springer 1978. 

A widespread and perhaps primitive function of the mandibular gland secretions is defense. Bees typically release these chemicals when they are roughly handled. The secretions include chemicals that are common arthropod defensive compounds. When crickets are treated with the mandibular gland compounds of Ceratina, attack by ants is deterred (Wheeler et al., 1977). 2-Heptanone in Apis worker mandibular glands acts as an alarm pheromone in conjunction with sting gland secretions. 

P. J. Weldon and J. F. Carroll, Vertebrate chemical defense Secreted and topically acquired deterrents of arthropods. Chapter 3, pp. 47-75, in Insect Repellents Principles, Methods, and Uses, ed. by M. Debboun, S. P. Frances, and D. Strickman, Boca Raton, FL CRC Press, 2007. 

Perhaps the most interesting arthropodan defensive compounds from the point of view of structural diversity are the alkaloids. While alkaloids had long been believed to arise only as a consequence of plant secondary metabolism, it has become apparent over the last few decades that arthropods are both prolific and innovative alkaloid chemists. The millipede Polyzonium rosalbum, once thought to secrete camphor (20), in fact gives off a camphoraceous/earthy aroma produced by the spirocyclic isoprenoid imine polyzonimine (21). 

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